I'm not using voice, just silent .MP3 files. Well, that's the problem. I tried making a silent .MP3 file but Audacity couldn't export it without the LAME MP3 Encoder. So I downloaded that, found its directory and tried using it, but it said it couldn't open it. So when Audacity told me to find it, I went to its directory, but it didn't appear.
Well, in my case, the length of the in-game dialogue text was relative to the length of the .lip file... That means a silent voice file would do no good. I don't know if this applies to you or anyone else, but in my case it really was the .lip file,,,
What comes to your Audacity problem, I can only say that that's odd. Try putting the lame_enc (or what ever it's name was) to a short location (like C:\LAME) and try again.